1. Field of the Invention
The present invention relates design of integrated circuits, and more specifically to a method and apparatus which generates an invariant (constant/fixed) and pre-specified (nominal or desired) voltage independent of fabrication process variations and changes in ambient temperature.
2. Related Art
Reference voltages are often generated using techniques, which produce a fixed voltage based on the bandgap voltage of silicon. In some prior embodiments, these reference voltages are generated by adding a term with a positive temperature coefficient (typically generated as a difference of base-emitter voltages of two bipolar junction transistors with unequal current densities) and the base-emitter voltage of a bipolar junction transistor. This reference voltage approaches the bandgap voltage of Silicon as the absolute temperature approaches 0 K, hence is called bandgap reference.
There is a recognized need that the reference voltage equal a pre-specified (desired/nominal) voltage, in addition to being invariant (does not change over time) in various operating conditions (e.g., fabrication process and ambient temperature). Example environments where such a need exists include, without limitation, analog-to-digital converters (ADCs), regulators, etc.
One known reason for a reference voltage to deviate from a pre-specified voltage is variations typically encountered in fabrication processes. In general, the variations lead to corresponding variations in the voltage level across the junction (providing the bandgap reference during operation), which may reflect in the reference voltage sought to be generated.
In one prior approach, components such as fuses and/or resistor networks are used, which can be configured (blowing the fuses or trimming the resistance) to ensure that the reference voltage equals a pre-specified value. However, such an approach generally leads to several disadvantages such as increase in the overall cost of the products (since the approach requires testing to determine the deviations from the pre-specified value), requiring additional area on the fabricated integrated circuit, etc.
What is therefore needed is a method and apparatus which generates an invariant (constant/fixed) and pre-specified (nominal or desired) voltage while meeting at least some of the requirements noted above.